1. Field of the Invention
The present invention relates to a sheet creaser, a sheet conveyer including a conveying path along which the sheet creaser is provided, a sheet finisher including the sheet creaser, an image forming apparatus including the sheet finisher or the sheet finisher.
2. Description of the Related Art
In the field of image forming apparatuses such as inkjet printers, electrophotographic copiers, facsimile machines, and multifunction products (MFPs), sheet finishers that receive a set of sheet-like recording mediums (hereinafter, “sheets”) from an image forming apparatus and perform post-processing such as stapling have been widely used. With the development of multi-functional sheet finishers, sheet finishers with both a side-stitch function and a saddle-stitch function have appeared. In most of the sheet finishers with the saddle-stitch function, a folding unit that folds the set of sheets includes at least one pair of rollers called pressure rollers and a plate member called folding plate. More particularly, the folding plate is aligned with a line to be folded of the set of sheets, and inserts the set of sheets into a nip between the pressure rollers. Thus, a crease is made along the line to be folded on the set of sheets with the nip.
Some folding units include a first pair of pressure rollers and a second pair of pressure rollers. The set of sheets is pressed twice with the first pressure rollers and the second pressure rollers, which makes a stronger crease.
However, even when the set of sheets is pressed twice, it is difficult to make a crease strong enough due to a short pressing time and a low pressing force. Because a rotation axis of the pressure rollers runs parallel to a direction perpendicular to a sheet conveying direction, a folded side of the set of sheets is pressed in the nip between the pressure rollers only for a short time. Moreover, because the pressure rollers nip the entire folded side at the same time, the pressing force on the set of sheets is distributed, i.e., the pressing force per unit area is low.
To solve the above problems, Japanese Patent Application Laid-open No. S62-16987 discloses a technology for making the stronger crease with a slidable pressure roller. More particularly, after the folding plate inserts the set of sheets into the nip between the pressure rollers and thereby the set of sheets is half-folded, the slidable pressure roller re-presses the folded side while sliding on the folded side in the direction perpendicular to the sheet conveying direction by an operation of a screw.
Because the folded side is pressed with the slidable pressure roller sliding in the direction perpendicular to the sheet conveying direction, the pressing force is applied only at one point of the folded side making a contact with the slidable pressure roller at a time. Because the slidable pressure roller slides on the folded side, the entire folded side is pressed with the high pressing force. As a result, the strong crease is made on the set of sheets. However, just when sliding up on the folded side, the slidable pressure roller may push the sheets in the sliding direction. As a result, the slidable pressure roller may fail to evenly press the entire length of the folded side.
Japanese Patent Application Laid-open No. 2005-162345 discloses a technology for making an evenly-pressed stronger crease by preventing such an undesired displacement of the sheets in the sliding direction caused by the pressing force when the slidable pressure roller slides up on the sheets. More particularly, the sheet finisher according to Japanese Patent Application Laid-open No. 2005-162345 includes a guiding unit, the slidable pressure roller, and a supporting member. The guiding unit guides a half-folded recording medium P from the pressure rollers to the slidable pressure roller. The slidable pressure roller re-presses the folded side of the recording medium P, sliding in the direction perpendicular to the sheet conveying direction. The supporting member supports side surfaces of the recording medium P during the slide-pressing performed by the slidable pressure roller.
The supporting member that supports the sides of the sheets during the slide-pressing of the folded side by the slidable pressure roller is effective in preventing the undesired displacement of the sheets in the sliding direction, and thereby the evenly-pressed stronger crease is made without fails. However, a setting space for the supporting member is required and a space for a mechanism for driving the supporting member increases. Therefore, it is disadvantageous from the viewpoint of downsizing.